1. Field of the Invention
The present invention relates to a light emitting diode used in LED display, back light source, traffic signal, trailway signal, illuminating switch, indicator, etc. More particularly, it relates to a light emitting device (LED) comprising a phosphor, which converts the wavelength of light emitted by a light emitting component and emits light, and a display device using the light emitting device.
2. Description of Related Art
A light emitting diode is compact and emits light of clear color with high efficiency. It is also free from such a trouble as burn-out and has good initial drive characteristic, high vibration resistance and durability to endure repetitive ON/OFF operations, because it is a semiconductor element. Thus it has been used widely in such applications as various indicators and various light sources. Recently light emitting diodes for RGB (red, green and blue) colors having ultra-high luminance and high efficiency have been developed, and large screen LED displays using these light emitting diodes have been put into use. The LED display can be operated with less power and has such good characteristics as light weight and long life, and is therefore expected to be more widely used in the future.
Recently, various attempts have been made to make white light sources by using light emitting diodes. Because the light emitting diode has a favorable emission spectrum to generate monochromatic light, making a light source for white light requires it to arrange three light emitting components of R, G and B closely to each other while diffusing and mixing the light emitted by them. When generating white light with such an arrangement, there has been such a problem that white light of the desired tone cannot be generated due to variations in the tone, luminance and other factors of the light emitting component. Also when the light emitting components are made of different materials, electric power required for driving differs from one light emitting diode to another, making it necessary to apply different voltages different light emitting components, which leads to complex drive circuit. Moreover, because the light emitting components are semiconductor light emitting components, color tone is subject to variation due to the difference in temperature characteristics, chronological changes and operating environment, or unevenness in color may be caused due to failure in uniformly mixing the light emitted by the light emitting components. Thus light emitting diodes are effective as light emitting devices for generating individual colors, although a satisfactory light source capable of emitting white light by using light emitting components has not been obtained so far.
In order to solve these problems, the present applicant previously developed light emitting diodes which convert the color of light which is emitted by light emitting components, by means of a fluorescent material disclosed in Japanese Patent Kokai Nos. 5-152609, 7-99345, 7-176794 and 8-7614. The light emitting diodes disclosed in these publications are such that, by using light emitting components of one kind, are capable of generating light of white and other colors, and are constituted as follows.
The light emitting diode disclosed in the above gazettes are made by mounting a light emitting component, having a large energy band gap of light emitting layer, in a cup provided at the tip of a lead frame, and having a fluorescent material that absorbs light emitted by the light emitting component and emits light of a wavelength different from that of the absorbed light (wavelength conversion), contained in a resin mold which covers the light emitting component.
The light emitting diode disclosed as described above capable of emitting white light by mixing the light of a plurality of sources can be made by using a light emitting component capable of emitting blue light and molding the light emitting component with a resin including a fluorescent material that absorbs the light emitted by the blue light emitting diode and emits yellowish light.
However, conventional light emitting diodes have such problems as deterioration of the fluorescent material leading to color tone deviation and darkening of the fluorescent material resulting in lowered efficiency of extracting light. Darkening here refers to, in the case of using an inorganic fluorescent material such as (Cd, Zn)S fluorescent material, for example, part of metal elements constituting the fluorescent material precipitate or change their properties leading to coloration, or, in the case of using an organic fluorescent material, coloration due to breakage of double bond in the molecule. Especially when a light emitting component made of a semiconductor having a high energy band gap is used to improve the conversion efficiency of the fluorescent material (that is, energy of light emitted by the semiconductor is increased and number of photons having energies above a threshold which can be absorbed by the fluorescent material increases, resulting in more light being absorbed), or the quantity of fluorescent material consumption is decreased (that is, the fluorescent material is irradiated with relatively higher energy), light energy absorbed by the fluorescent material inevitably increases resulting in more significant degradation of the fluorescent material. Use of the light emitting component with higher intensity of light emission for an extended period of time causes further more significant degradation of the fluorescent material.
Also the fluorescent material provided in the vicinity of the light emitting component may be exposed to a high temperature such as rising temperature of the light emitting component and heat transmitted from the external environment (for example, sunlight in case the device is used outdoors).
Further, some fluorescent materials are subject to accelerated deterioration due to combination of moisture entered from the outside or introduced during the production process, the light and heat transmitted from the light emitting component.
When it comes to an organic dye of ionic property, direct current electric field in the vicinity of the chip may cause electrophoresis, resulting in a change in the color tone.